The purpose of this study is to investigate how students' high school experience, math and science efficacy, and student, parent, and teacher expectations affect their plans for college major choice after controlling for students' gender, ethnicity, and parental variables. Over 1500 9th grade students participated in the study. Using logistic regressions, we found that males and students whose parents held degree from a U.S. college are more likely to consider STEM majors in college. Hispanic students were found less likely to consider STEM major in college compared their Asian counterparts. Students who completed more STEM PBL projects and attended STEM summer camps are more likely to consider STEM majors. Students with higher GPAs also indicated that they are more likely to study STEM majors in college. In addition, students with higher parent and teacher encouragement are more likely to consider selecting a STEM major after graduating from high school. Moreover, students who had higher math and science efficacy are also more likely to consider choosing a STEM major in college. Last but not least, we found that students' future career choice is also positively associated with their interests and goals they develop during high school years. Other findings and interaction effects with gender and ethnicity are also discussed in the paper. Overall, this study demonstrates that students' contemplations about STEM major selection in college is influenced by the complex interplay between the "individual," "environment," and "behavior," three major components of social cognitive career theory.